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Chapter 17. KVM guest timing management

Virtualization poses various challenges for guest time keeping. Guests which use the Time Stamp Counter (TSC) as a clock source may suffer timing issues as some CPUs do not have a constant Time Stamp Counter. Guests without accurate timekeeping may have issues with some networked applications and processes as the guest will run faster or slower than the actual time and fall out of synchronization.
KVM works around this issue by providing guests with a para-virtualized clock. Alternatively, some guests may use other x86 clock sources for their timing in future versions of those operating systems.
Presently, only Red Hat Enterprise Linux 5.4 and newer guests fully support the para-virtualized clock.
Guests can have several problems caused by inaccurate clocks and counters:
  • Clocks can fall out of synchronization with the actual time which invalidates sessions and affects networks.
  • Guests with slower clocks may have issues migrating.
These problems exist on other virtualization platforms and timing should always be tested.

Important

The Network Time Protocol (NTP) daemon should be running on the host and the guests. Enable the ntpd service:
# service ntpd start
Add the ntpd service to the default startup sequence:
# chkconfig ntpd on
Using the ntpd service should minimize the affects of clock skew in all cases.
Determining if your CPU has the constant Time Stamp Counter

Your CPU has a constant Time Stamp Counter if the constant_tsc flag is present. To determine if your CPU has the constant_tsc flag run the following command:

$ cat /proc/cpuinfo | grep constant_tsc
If any output is given your CPU has the constant_tsc bit. If no output is given follow the instructions below.
Configuring hosts without a constant Time Stamp Counter

Systems without constant time stamp counters require additional configuration. Power management features interfere with accurate time keeping and must be disabled for guests to accurately keep time with KVM.

Important

These instructions are for AMD revision F cpus only.
If the CPU lacks the constant_tsc bit, disable all power management features (BZ#513138). Each system has several timers it uses to keep time. The TSC is not stable on the host, which is sometimes caused by cpufreq changes, deep C state, or migration to a host with a faster TSC. Deep C sleep states can stop the TSC. To prevent the kernel using deep C states append "processor.max_cstate=1" to the kernel boot options in the grub.conf file on the host:
term Red Hat Enterprise Linux Server (2.6.18-159.el5)
        root (hd0,0)
	kernel /vmlinuz-2.6.18-159.el5 ro root=/dev/VolGroup00/LogVol00 rhgb quiet processor.max_cstate=1
Disable cpufreq (only necessary on hosts without the constant_tsc) by editing the /etc/sysconfig/cpuspeed configuration file and change the MIN_SPEED and MAX_SPEED variables to the highest frequency available. Valid limits can be found in the /sys/devices/system/cpu/cpu*/cpufreq/scaling_available_frequencies files.
Using the para-virtualized clock with Red Hat Enterprise Linux guests

For certain Red Hat Enterprise Linux guests, additional kernel parameters are required. These parameters can be set by appending them to the end of the /kernel line in the /boot/grub/grub.conf file of the guest.

The table below lists versions of Red Hat Enterprise Linux and the parameters required for guests on systems without a constant Time Stamp Counter.
Red Hat Enterprise LinuxAdditional guest kernel parameters
5.4 AMD64/Intel 64 with the para-virtualized clockAdditional parameters are not required
5.4 AMD64/Intel 64 without the para-virtualized clocknotsc lpj=n
5.4 x86 with the para-virtualized clock Additional parameters are not required
5.4 x86 without the para-virtualized clockclocksource=acpi_pm lpj=n
5.3 AMD64/Intel 64notsc
5.3 x86clocksource=acpi_pm
4.8 AMD64/Intel 64notsc
4.8 x86clock=pmtmr
3.9 AMD64/Intel 64 Additional parameters are not required
3.9 x86Additional parameters are not required

Warning

The divider kernel parameter was previously recommended for Red Hat Enterprise Linux 4 and 5 guests that did not have high responsiveness requirements, or exist on systems with high guest density. It is no longer recommended for use with guests running Red Hat Enterprise Linux 4, or Red Hat Enterprise Linux 5 versions prior to version 5.8.
divider can improve throughput on Red Hat Enterprise Linux 5 versions equal to or later than 5.8 by lowering the frequency of timer interrupts. For example, if HZ=1000, and divider is set to 10 (that is, divider=10), the number of timer interrupts per period changes from the default value (1000) to 100 (the default value, 1000, divided by the divider value, 10).
BZ#698842 details a bug in the way that the divider parameter interacts with interrupt and tick recording. This bug is fixed as of Red Hat Enterprise Linux 5.8. However, the divider parameter can still cause kernel panic in guests using Red Hat Enterprise Linux 4, or Red Hat Enterprise Linux 5 versions prior to version 5.8.
This parameter was not implemented in Red Hat Enterprise Linux 3, so this bug does not affect Red Hat Enterprise Linux 3 guests.
Red Hat Enterprise Linux 6 does not have a fixed-frequency clock interrupt; it operates in tickless mode and uses the timer dynamically as required. The divider parameter is therefore not useful for Red Hat Enterprise Linux 6, and Red Hat Enterprise Linux 6 guests are not affected by this bug.
Using the Real-Time Clock with Windows Server 2003 and Windows XP guests

Windows uses the both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For Windows guests the Real-Time Clock can be used instead of the TSC for all time sources which resolves guest timing issues.

To enable the Real-Time Clock for the PMTIMER clocksource (the PMTIMER usually uses the TSC) add the following line to the Windows boot settings. Windows boot settings are stored in the boot.ini file. Add the following line to the boot.ini file:
/use pmtimer
For more information on Windows boot settings and the pmtimer option, see Available switch options for the Windows XP and the Windows Server 2003 Boot.ini files.
Using the Real-Time Clock with Windows Vista, Windows Server 2008 and Windows 7 guests

Windows supports both the Real-Time Clock (RTC) and the Time Stamp Counter (TSC). For some Windows guests, the RTC can be selected for use instead of the TSC for all of the guest's time sources. This can resolve some guest timing issues.

Note

The boot.ini file is no longer used in Windows Vista and newer. As shown in this procedure, Windows Vista, Windows Server 2008 and Windows 7 use the Boot Configuration Data Editor (bcdedit.exe) application to modify this boot parameter.

Important

This procedure is only recommended if the guest is having time keeping issues, and is not supported in Windows Server 2008 or Windows Server 2008 SP2 as they do not support the USEPLATFORMCLOCK parameter demonstrated here and already use the RTC by default. This procedure is however supported in Windows Server 2008 R2.
  1. Open the Windows guest.
  2. In the guest, open the Accessories menu of the start menu. Right click on the Command Prompt application, and select Run as Administrator.
  3. Confirm any security exception, if prompted.
  4. Set the boot manager to use the RTC (platform clock) for the primary clock source. The system UUID ({default} in the following example) should be changed if your system UUID is different than the default boot device.
    C:\Windows\system32>bcdedit /set {default} USEPLATFORMCLOCK on
    The operation completed successfully
Time keeping for Windows Vista, Windows Server 2008 R2 and Windows 7 guests should now be improved.